Canfieldite, Ag8SnS6, is a semiconducting mineral notable for its high ionic conductivity, photosensitivity, and low thermal conductivity. We report the solution growth of large single crystals of Ag8SnS6 of mass up to 1 g from a ternary Ag-Sn-S melt. On cooling from high temperature, Ag8SnS6 undergoes a known cubic (F-43m) to orthorhombic (Pna21) phase transition at ≈ 460 K. By studying the magnetization and thermal expansion between 5-300 K, we discover a second structural transition at ≈ 120 K. Single crystal X-ray diffraction reveals the low temperature phase adopts a different orthorhombic structure with space group Pmn21 (a = 7.6629(5) \AA, b = 7.5396(5) \AA, c = 10.6300(5) \AA, Z = 2 at 90 K) that is isostructural to the room temperature forms of the related Se-based compounds Ag8SnSe6 and Ag8GeSe6. The 120 K transition is first-order and has a large thermal hysteresis. Based on magnetization and thermal expansion data, the room temperature polymorph can be kinetically arrested into a metastable state by rapidly cooling to temperatures below 40 K. We lastly compare the room and low temperature forms of Ag8SnS6 with its argyrodite analogues, Ag8TQ6 (T = Si, Ge, Sn; Q = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller phase volume favors the Pna21 arrangement. We support this picture by showing that the transition to the Pmn21 phase is avoided in Ge alloyed Ag8Sn1−xGexS6 samples as well as pure Ag8GeS6.
@article{arxiv.2110.07508,
title = {A Low Temperature Structural Transition in Canfieldite, Ag$_8$SnS$_6$, Single Crystals},
author = {Tyler J. Slade and Volodymyr Gvozdetskyi and John M. Wilde and Andreas Kreyssig and Elena Gati and Lin-Lin Wang and Yaroslav Mudryk and Raquel A. Ribeiro and Vitalij K. Pecharsky and Julia V. Zaikina and Sergey L. Budko and Paul C. Canfield},
journal= {arXiv preprint arXiv:2110.07508},
year = {2021}
}